Muting circuit for an FM radio receiver

ABSTRACT

Signal transmitting operation is effected in a muting circuit when a first constant current source connected to the emitters of first and second transistors is operative with a second constant current source connected to the emitters of third and fourth transistors being inoperative. The signal transmitting operation is not effected in the opposite case. A bias resistor is interposed between the bases of the first and third transistors. To eliminate the popping noise when the power source is turned on, a switching element is connected parallel to the bias resistor and is kept in the ON state for a predetermined period of time after making of the power source. 
     In an FM radio receiver, the output signal of a detuning detection circuit is applied to the input terminals of first and second mute control circuits, and the muting operation of a pre-amplifier for amplifying a stereo composite signal is controlling by the first mute control circuit while the muting operation of a muting circuit connected to the output of a stereo demodulation circuit is controlled by the second mute control circuit. Discrimination levels of these first and second mute control circuits are set to mutually different levels to eliminate occurrence of the popping noise at the time of changes from tuning to detuning and vice versa.

BACKGROUND OF THE INVENTION

This invention relates to a muting circuit and to an FM radio receiver having the muting circuit.

As a muting circuit providing a large signal attenuation, the inventors of the present invention previously proposed a so-called "electronic switching circuit" making use of a differential transistor circuit such as shown in FIG. 1, for example.

In this circuit, collectors of differential amplification transistors Q₁, Q₂ for signal transmission and those of differential transistors Q₃, Q₄ for forming a d.c. signal at the time of muting operation are commonly connected, and constant current transistors Q₅, Q₆ are connected to common emitters of the respective differential transistors Q₁, Q₂ and to those of Q₃, Q₄, the constant current transistors Q₅, Q₆ being selectively and alternately operated by a switch SW₁.

An input signal is applied to the base of the differential transistor Q₁. A bias voltage V_(B1) is also applied to the base of transistor Q₁ via a bias resistor R_(B), and to the base of the differential transistor Q₃ directly. A negative feedback signal is applied to the bases of the transistors Q₂, Q₄ via a feedback circuit β, thereby setting the gain of the differential amplification transistor circuit and providing a d.c. bias voltage.

The circuit operates as follows: When the switch SW₁ is set to side a, the constant current transistor Q₅ becomes inoperative while the constant current transistor Q₆ becomes operative so that the differential amplification transistors Q₁, Q₂ are turned off while the differential transistors Q₃, Q₄ are turned on. Consequently, only the same d.c. signal as the bias voltage V_(B1), that is formed by the differential transistors Q₃, Q₄, can be obtained at the output OUT.

On the other hand, when the switch SW₁ is set to side b, the differential amplification transistors Q₁, Q₂ become operative and transmit the input signal IN. Accordingly, a large signal attenuation can be obtained without changing the output d.c. signal.

The inventors of the present invention found that in this muting circuit when the input signal IN is applied via a coupling capacitor C₁ the following problem occurs.

Namely, it is possible to assume that the input terminal IN in FIG. 1 is substantially grounded through the practically negligible impedance of a signal source (not shown), so that the bias voltage V_(B1) rises immediately at the time t₁, immediately after connecting the power source as shown in FIG. 2, whereas the voltage V_(B1), at the junction between the coupling capacitor C₁ and the bias resistor R_(B) gradually rises due to the time constant of capacitor C₁ and resistor R_(B).

Accordingly, if the switch SW₁ is kept closed to the side a during a predetermined period from the time t₁ to the time t₂, the base voltage of both differential amplification transistors Q₃, Q₄ is equal to the bias voltage V_(B1) and consequently, the collector potential of the transistor Q₄ becomes V_(B1) /β. Since an output coupling capacitor C_(o) connected to the output terminal OUT and the impedance of a load (not shown) form a differential circuit together, the waveform obtained by differentiating the collector potential V_(B1) /β of the transistor Q₄ occurs at the output terminal OUT at the time t.sub., as depicted in the waveform diagram of FIG. 2. This transient response waveform occurring at the time t₁ eventually results in a popping noise, but can be prevented by arranging a muting circuit (not shown) at a subsequent stage of this circuit.

When the switch SW₁ is switched from side a to side b at the time t₂, the base of the transistor Q₂ is biased by the bias voltage V_(B1) whereas the base of transistor Q₁ is biased by the voltage V_(B1) ' which is lower than the bias voltage V_(B1). Consequently, since a voltage difference ΔV as shown in the waveform diagram of FIG. 2 is applied between the bases of the differential amplification transistors Q₁ and Q₂, the collector potential of the transistor Q₂ drops drastically at the time t₂. The waveform obtained by differentiating this collector potential by the output coupling capacitor C_(o) and the impedance of the load occurs at the output terminal OUT at the time t₂ as shown in the waveform diagram of FIG. 2.

The inventors of the present invention have found that prevention of the popping noise at the time t₂ can be accomplished by reducing the time constant at the rise of the voltage V_(B1) ' to an extremely small value. Accordingly, the inventors examined the idea of reducing the capacitance of the coupling capacitor C₁, but this plan was abandoned because a problem of phase inversion of low frequency signals arises.

Reducing the resistance of the bias resistor R_(B) was attempted, but this plan was also abandoned because of the lowering of the input impedance at the input terminal IN.

Occurrence of the popping noise due to the transient response waveform at the time t₂ can be prevented by disposing another muting circuit (not shown) at a subsequent stage of this circuit, but when the capacitance of the coupling capacitor and the resistance of the bias resistor R_(B) are set to about 10 μF and about 40 KΩ, respectively, the muting time of the muting circuit disposed at the subsequent stage of this circuit must be set to as long as about three seconds. If the muting operation is carried out for such a relatively long period after turning on the power source, however, the listener of the radio receiver would think that the radio receiver was out of order.

On the other hand, as one of the additional functions of an FM radio receiver, a muting circuit for eliminating offensive noise between stations during selection of the stations has heretofore been known, as disclosed in "HI-FI FM TUNER", p. 193-199, published on Aug. 20, 1976 by Nippon Hoso Kyokai, for example.

FM radio receivers employing this muting circuit in a monolithic semiconductor integrated circuit for FM intermediate frequency amplification and detection are commercially available from RCA, U.S.A., under the tradename "CA3089E", and from Hitachi Limited, Japan, under the tradename "HA1137W".

FIG. 3 shows a block diagram of an FM radio receiver including the monolithic semiconductor integrated circuit. This radio receiver was examined by the inventors of the present invention prior to the filing of this patent application.

In the drawing, symbol IC₁ represents the monolithic semiconductor integrated circuit for FM intermediate frequency amplification and detection. This integrated circuit includes an FM intermediate frequency amplification stage 1, an FM detection stage 2, a low frequency amplication stage 3, a detuning detection circuit 4, and a mute control circuit 6 for controlling the muting operation at the low frequency amplification stage 3. Additionally, this monolithic semiconductor integrated circuit includes an AFC circuit and a level detection circuit, the output signals of which are used as the input signals to the detuning detection circuit 4. Reference numeral 5 represents a time constant circuit which is disposed for removing high frequency components contained in the detuning detection signal.

Symbol IC₂ represents a monolithic semiconductor integrated circuit for FM stereo demodulation. This integrated circuit includes a pre-amplifier 7, a PLL (phase locked loop) circuit 8 for forming a 38 KHz switching signal synchronous with a 19 KHz pilot signal contained in a stereo composite signal, and a stereo demodulation circuit 9. The stereo composite signal formed by the FM detection circuit 2 is applied to the input of the pre-amplifier 7 via a coupling capacitor C₁₀₀ while the stereo demodulation circuit 9 forms stereo demodulated signals L, R of the left and right channels by receiving at its input the stereo composite signal passing through the pre-amplifier 7 and the 38 KHz switching signal formed by the PLL circuit 8.

The monolithic semiconductor integrated circuit IC₂ also includes additional circuits for stereo-monaural detection, selection, display, and the like.

In the above monolithic semiconductor integrated circuit IC₁, the popping noise V_(pop) originates from the FM detection stage 2 due to changes in the d.c. voltage in the S characteristic curve of the FM detection circuit 2 as shown in FIG. 4 during the muting operation for changing over from tuning to detuning upon detection of detuning.

Accordingly, the inventors of the present invention examined the possibility of providing a muting circuit 10 at the output of the stereo demodulation circuit 9 as the subsequent stage. In such a case, the muting circuit 10 must exhibit fast mute-release timing for shifting from detuning to tuning and retarded mute timing for shifting from tuning to detuning in its muting operation in the monolithic semiconductor integrated circuit IC₁. Since these muting operations are independently controlled, the timings are difficult to set and are highly susceptible to variance of element characteristics.

Moreover, the coupling capacitor C₁₀₀ is interposed between the two muting circuits 6 and 10 and causes a time advance of the signal to the subsequent muting circuit, so that it becomes extremely difficult for the subsequent muting circuit 10 to eliminate the popping noise generated in the preceding muting circuit 6.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a muting circuit which prevents the occurrence of a popping noise at the time of mute release after turning on the power source.

It is another object of the present invention to provide an FM radio receiver which is capable of reliably eliminating the popping noise.

In accordance with a fundamental characteristic feature of the first invention of this application, in a muting circuit making use of an electronic switching circuit, a bias resistor disposed at the base of an amplification transistor for signal transmission is short-circuited by a switching means for a predetermined period from immediately after turning on the power source until the muting operation is released.

In accordance with a fundamental characteristic feature of the second invention of this application, mute control circuits receiving the same tuning or detuning detection signal and having mutually different discrimination levels are employed so that the operation start timing of the muting circuit at the later stage is set earlier than that of the muting circuit at the prior stage whereas the operation release timing of the later stage is set later than that of the prior stage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of the muting circuit that was proposed by the inventors of the present invention prior to the present application;

FIG. 2 is an operation waveform diagram of the circuit of FIG. 1;

FIG. 3 is a block diagram of the FM radio receiver that was proposed by the inventors of the present invention prior to the present application;

FIG. 4 is an operative waveform diagram of the receiver of FIG. 3;

FIG. 5 is a circuit diagram in accordance with an embodiment of the first invention;

FIG. 6 is an operative waveform diagram of the embodiment of FIG. 5;

FIG. 7 is a circuit diagram in accordance with another embodiment of the first invention;

FIG. 8 is a block diagram of an FM radio receiver in accordance with an embodiment of the second invention;

FIG. 9 is an operative waveform diagram of the embodiment of FIG. 8; and

FIG. 10 is a circuit diagram showing the principal portions of the second invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in detail with reference to embodiments thereof.

FIG. 5 shows a circuit diagram of an embodiment of the first invention of this application.

In this circuit, the collectors of differential amplification transistors Q₁, Q₂ for signal transmission and those of the differential transistors Q₃, Q₄ for forming the d.c. signal at the time of the muting operation are connected together as in the aforementioned circuit, and the constant current transistors Q₅, Q₆ that are alternately operated by the switch SW₁ are connected to the common emitters of the respective differential transistors Q₁, Q₂ and Q₃, Q₄.

The bias voltage V_(B1) is applied to the base of the differential amplification transistor Q₁, to which the input signal IN is also applied, via the bias resistor R_(B), while the bias voltage V_(B1) is applied to the base of the differential transistor Q₃ directly. A negative feedback signal is applied to the bases of the transistors Q₂, Q₄ via the feedback circuit β, thereby setting the gain of the differential amplification transistor circuit and providing the d.c. bias voltage.

The load resistor R_(L) is connected to the collectors of the transistors Q₂, Q₄ and also to the output terminal OUT. An input signal IN is applied to the base of the transistor Q₁ via the coupling capacitor C₁.

In the circuit of this embodiment, a switching transistor Q₇ is disposed in parallel to the bias resistor R_(B) in order to prevent occurrence of the popping noise at the time of mute release after turning on the power source, and is controlled by a voltage V applied via a switch SW₂. This switch SW₂ is connected to side a', for example, when the power source is applied, thereby turning on the transistor Q₇. The transistor Q₇ is kept ON until the switch SW₁ is changed over to the side b or before the release of the muting operation, and is turned off after the release of the muting operation. It is therefore preferred that both switches SW₁ and SW₂ be operated in an interlocking arrangement by the same control signal.

The muting operation by the switch SW₁ when the power source in this circuit is turned on is the same as in the aforementioned circuit, and so its explanation is deleted. In this embodiment, the bias resistor R_(B) is short-circuited by the transistor Q₇ turned on by the switch SW₂ immediately after the power source is turned on. As shown in the operative waveform diagram of FIG. 6, therefore, the base voltage V_(B1) ' of the differential amplification transistor Q₁ substantially follows the bias voltage V_(B1), which rises drastically immediately after the power source is turned on (at the time t₁). Accordingly, when the switch SW₁ is changed over to side b at time t₂ to release the muting operation, both bias voltages V_(B1) and V_(B1) ' become equal to each other so that occurrence of the popping noise can be prevented.

Incidentally, the voltage fluctuation occurring at the time t₁ immediately after turning on the power source (or the popping noise) can be eliminated by the muting circuit of the subsequent stage which also functions to protect the speaker connected to a power amplifier that is simultaneously turned on. As described above, however, when the popping noise occurs at the time t₂, the muting operation of the muting circuit at the subsequent stage has already released so that the amplification circuit at the subsequent stage is operative and the voltage fluctuation is amplified as a loud popping noise.

FIG. 7 shows a circuit in accordance with an embodiment of the first invention.

The circuit of this embodiment includes circuit elements that are formed on a single chip silicon substrate in accordance with the known fabrication method of a semiconductor device. However, a capacitor C₁, a resistor R₂₀ and a capacitor C₃ connected via terminals P₁, P₂ are formed by external components. The muting circuit of this embodiment also functions as a pre-amplifier for a stereo demodulation circuit or the like in an FM stereo tuner or the like, for example.

Accordingly, the differential amplification transistor circuit for signal transmission is composed of transistors Q₁, Q₁ ' and Q₂, Q₂ ' in the Darlington circuit arrangement in order to realize high input impedance and to increase open loop gain, whereby the emitters of the transistors Q₁ ', Q₂ ' on the input side are furnished with constant current source circuits formed by transistors Q₁₁, Q₁₂ and resistors R₆, R₇, respectively. The emitters of the transistors Q₁, Q₂ are connected together via emitter resistors R₁, R₂, and a resistor R₃ is disposed between the junction of these resistors and the collector of the constant current transistor Q₅.

A similar resistor R₄ is disposed between the collector of the constant current transistor Q₆ and the emitters of the differential transistors Q₃, Q₄ which are connected to each other.

A current mirror circuit is disposed at the collectors of the amplification transistors Q₁, Q₂ as a load, and is composed of transistors Q₈, Q₉.

This amplified output is applied to an emitter follower output circuit consisting of a transistor Q₁₀ and a resistor R₅.

An output signal is obtained from its output terminal OUT and is fed back to the bases of the transistors Q₄ and Q₂ '. A capacitor C₂ for preventing oscillation or the like is connected to the base of the emitter follower transistor Q₁₀, the capacitor being an MOS (metal oxide semiconductor) capacitor, for example.

A resistor R_(B) ' for preventing electrostatic breakdown or the like is connected to the base of the input amplification transistor Q₁ '.

In this embodiment, an example of a concretely realized circuit of the switches SW₁, SW₂ is illustrated.

When the power source is turned on, an integration voltage from the RC circuit R₂₀, C₃ connected to a terminal P₂ is applied to the base of the transistor Q₁₃ via a resistor R₁₁. Level shift diodes Q₁₄, Q₁₅ (the term "diode" will hereinafter include transistors of a diode-type) and a resistor R₁₃ are connected in series to the emitter of transistor Q₁₃. The voltage drop across resistor R₁₃ is applied to the base of the transistor Q₁₆ via resistor R₁₂. Resistor R₁₀ is interposed between the collector of this transistor Q₁₆ and a power source voltage V_(cc2), which is higher than the bias voltage V_(B1). The collector voltage of transistor Q₁₆ is applied to the base of transistor Q₇ as its controlling signal.

Accordingly, the transistor Q₁₆ and the resistor R₁₀ together form the switch SW₂.

The voltage drop across resistor R₁₃ is applied to the base of the transistor Q₁₇, and a diode Q₁₈ is connected to the emitter of this transistor Q₁₇ as a constant voltage source. Further, a resistor R₁₄ is disposed between the collector of transistor Q₁₇ and the stabilized power source voltage V_(cc1).

On the other hand, a voltage from the power source voltage V_(cc1) which has been level-shifted by level shift diodes Q₂₁, Q₂₂ and a resistor R₁₉ is applied to the base of a transistor Q₂₀ while the collector voltage of transistor Q₁₇ is applied to the base of a transistor Q₁₉, and the emitters of these transistors Q₁₉, Q₂₀ are connected to each other and to the power source voltage V_(cc1) via a resistor R₁₆.

These transistors Q₁₉, Q₂₀ are p-n-p transistors whose collectors are equipped with resistors R₁₇, R₁₈, respectively. The collector voltages of these transistors Q₁₉, Q₂₀ are applied as the controlling signals to the bases of the constant current transistors Q₅, Q₆ via the resistors R₈, R₉, respectively. Accordingly, these circuit elements form the switch SW₁.

The operation of these switches SW₁, SW₂ is as follows.

During the period from immediately after the power source is turned on until its integrated voltage at the terminal P₂ reaches 4 V_(BE) (base-emitter voltage of the transistor or forward voltage of the diode), the transistor Q₁₆ is kept off so that the base voltage of the transistor Q₇ is as high as the power source voltage V_(cc2), and hence the transistor Q₇ is turned on.

When the voltage exceeds the abovementioned voltage 4 V_(BE), the transistor Q₁₆ is turned on so that the transistor Q₇ is turned off.

On the other hand, since the transistor Q₁₇ is kept off during the period before the abovementioned integration voltage reaches 5 V_(BE), the differential transistor Q₁₉ is kept off while the differential transistor Q₂₀ is kept on. Accordingly, the constant current transistor Q₆ is operative while the constant current transistor Q₅ is inoperative, thereby performing the same operation as when switch SW₁ is set to side a. Thus, the muting operation is carried out.

When the voltage at the terminal P₂ exceeds 5 V_(BE), the transistor Q₁₇ is turned on so that the differential transistor Q₁₉ is turned on with the differential transistor Q₂₀ being turned off. Consequently, the constant current transistor Q₅ is operative while the constant current transistor Q₆ is inoperative, thereby performing the same operation as when switch SW₁ is set to side b. Hence, muting is released and signal transmission is started.

Accordingly, the circuit of this embodiment is constructed so that the switching transistor Q₇ is turned off before the muting operation is released.

The first invention of the present application is not limited, in particular, to the abovementioned embodiment, and the definite example of the circuit for switches SW₁, SW₂ can be modified in various ways. It is possible, for example, to employ a construction in which the constant current transistors Q₅, Q₆ are compulsively and alternatively turned off when the switching transistors are turned on.

As the switching means for short-circuiting the base bias resistor R_(B), an MOSFET or the like may also be employed.

The first invention of this application can be widely applied as a muting circuit in a variety of audio equipment and devices.

FIG. 8 is a block diagram showing the principal portions of an embodiment of an FM radio receiver in accordance with the second invention of the present application.

Symbol IC₁ " represents an FM intermediate frequency amplification-detection monothilic semiconductor integrated circuit which includes an FM intermediate frequency amplification stage 1, an FM detection stage 2, a low frequency amplification stage 3 and a detuning detection circuit 4 and in which the mute control circuit is deleted in the low frequency amplification stage 3. Except in that the mute control circuit is deleted, this integrated circuit is the same as the aforementioned monolithic semiconductor integrated circuit IC₁ for FM intermediate frequency amplification and detection. However, the aforementioned commercial product "HA1137W" can be used as the monolithic semiconductor integrated circuit IC₁ " for FM intermediate frequency amplification and detection in this embodiment by rendering its mute control circuit 6 inoperative.

Symbol IC₂ " represents a monolithic semiconductor integrated circuit for FM stereo demodulation. Except those points which will hereinafter be described, this circuit is analogous to the aforementioned monolithic semiconductor integrated circuit IC₂ such as one disclosed in the magazine "ELECTRONICS", pages 62-66, November, 1971 or to a circuit available commercially from Hitachi Limited under the tradename "HA1196", or the like.

In the monolithic semiconductor integrated circuit IC₂ " for FM stereo demodulation in this embodiment, the muting function is added to the pre-amplifier 7, and a first mute control circuit 11 for controlling this muting function as well as a second mute control circuit 12 for controlling a muting circuit 10 disposed in the stereo demodulation output are additionally disposed. A detection signal formed by the detuning detection circuit 4 is applied to these mute control circuits 11, 12 together, and different detection levels V_(L1), V_(L2) are provided to discriminate necessary mute control.

Namely, as shown in FIG. 9, the discrimination level V_(L1) of the first mute control circuit 11 is set to the high level while the discrimination level V_(L2) of the second mute control circuit 12 is set to the low level so as to read the level difference from a detuning detection signal V_(M) of the detuning detection circuit 4, which becomes high level when the tuning frequency deviates excessively from the center frequency f_(o) of the S characteristic of the FM detection stage 2 and becomes low level when tuning is established.

Accordingly, when detuning changes to tuning and the muting operation is released, the detuning detection signal V_(M) reaches a level below the first discrimination level V_(L1) so that the muting operation in the pre-amplifier 7 is released before the muting operation of the muting circuit 10 is released. In this instance, the voltage fluctuation V_(pop) occurs due to the change in the d.c. voltage in the S characteristic but since the muting operation in the muting circuit 10 is not yet released, the occurrence of the popping noise is eliminated.

On the other hand, when tuning changes to detuning and the muting operation starts, the detuning detection signal V_(M) first reaches a level above the second discrimination level V_(L2) so that muting of the pre-amplifier 7 is started after the muting operation of the muting circuit 10. Accordingly, the popping noise occurring for the reason noted above is eliminated by muting circuit 10 which has already started its muting operation.

In this embodiment, since the mute control circuits 11, 12 are constructed by the same monolithic semiconductor integrated circuit IC₂, the discrimination levels V_(L1), V_(L2) for restricting their mute control operation are not affected by variance of the semiconductor elements, and consequently the mute control operation can be set to a high level of accuracy as described above.

Since no circuit element for shortening the signal transmission time such as a coupling capacitor C₁₀₀ is interposed between the muting circuit in the pre-amplifier 7 and the muting circuit 10 for the stereo demodulation circuit 9, it is possible to make the timing difference for the start and release of the muting operation, corresponding to the level difference of the discrimination levels V_(L1), V_(L2), coincide with the time difference for the elimination of the popping noise.

In other words, the popping noise can reliably be eliminated by a simple circuit design in which the level difference between V_(L1) and V_(L2) is set in accordance with the pulse width of the popping noise.

Incidentally, sufficient attenuation to eliminate noises between broadcasting stations can not be obtained by use of merely the muting circuit 10. Hence, it is necessary to dispose two muting circuit as described above.

FIG. 10 is a circuit diagram of an embodiment of this second invention.

In the drawing, the circuit elements of the portion IC₂ ' encircled by dotted lines are formed on one silicon semiconductor chip by the known fabrication method of semiconductors, and encircled numerals represent terminal numbers.

In the pre-amplifier 7, differential amplification transistors Q₁, Q₂ and Q₃, Q₄ for the signal transmission of the input signal IN from the low frequency amplification stage 3 are connected to each other in a Darlington arrangement with their collectors respectively connected together, while the collectors of differential trasistors Q₁₀, Q₁₁ for forming the d.c. signal at the time of the muting operation are connected, respectively, to the collectors of the former. Constant current circuits consisting of transistors Q₆, Q₇ and resistors R₅, R₆, respectively, are connected to the emitters of the input transistors Q₁, Q₃ in the Darlington configuration, respectively, and the bias constant voltage V_(B2) is applied to the bases of these transistors Q₆, Q₇. The emitters of the transistors Q₂, Q₄ are connected to each other via emitter resistors R₁, R₂, respectively, and a resistor R₃ is disposed between the junction of these resistors and the collector of the constant current transistor Q₅. A similar resistor R₄ is disposed between the emitters of the differential transistors Q₁₀, Q₁₁, that are connected to each other, and the collector of the constant current transistor Q₁₂.

A current mirror circuit consisting of transistors Q₈, Q₉ is connected as a load to the mutually connected collectors of the differential amplification transistors Q₁, Q₂ and Q₃, Q₄ and to the mutually connected connectors of the differential transistors Q₁₀, Q₁₁.

This amplification output is applied to an emitter follower output circuit consisting of a transistor Q₁₃ and a resistor R₉. A negative feedback signal is obtained from the output of the emitter follower output circuit to be fed back to the bases of the trasistors Q₃, Q₁₁ and an output signal (composite signal V_(CON)) to be applied to a stereo demodulation circuit 9 that will be described elsewhere. The composite signal is produced as an output from a second terminal via an inverting amplifier circuit consisting of a transistor Q₁₄, a collector resistor R₁₀ and an emitter resistor R₁₁, and via an emitter follower circuit consisting of a transistor Q₁₅ and an emitter resistor R₁₂.

A capacitor C₁ for preventing oscillation or the like is connected to the base of the abovementioned transistor Q₁₃, and this capacitor could composed of an MOS (metal oxide semiconductor) capacitor, for example.

The base bias voltage V_(B1) is applied to the base of the input differential amplification transistor Q₁ via a base bias resistor R₇ and is applied to the base of the differential transistor Q₁₀ directly.

The base of the abovementioned transistor Q₁ is connected to the first terminal via a resistor R₈ which is disposed for preventing dielectric breakdown. A composite input signal IN (in the case of the stereo broadcasting) is applied to this first terminal as an FM detection output signal through a coupling capacitor C₁₀₀. Reference numeral 13a represents a bias circuit which forms the abovementioned bias voltages V_(B1), V_(B2).

In this pre-amplifier 7, either one of the constant current transistors Q₅, Q₁₂ is alternatively actuated in order carry out the signal transmission of the input signal IN or the muting operation.

Namely, when the constant current transistor Q₅ is operative with the constant current transistor Q₁₂ kept off, the differential transistors Q₁₀, Q₁₁ are turned off while the differential amplification transistors Q₂, Q₄ operate so that the input signal IN is transmitted. In the opposite case, the differential amplification transistors Q₂, Q₄ are turned off with the differential transistors Q₁₀, Q₁₁ being operative so that the signal transmission of the input signal IN is inhibited and muting is carried out. In this case, only the same d.c. signal as the bias voltage V_(B1) is obtained at the emitter output of the transistor Q₁₃ due to the operation of the differential transistors Q₁₀, Q₁₁.

In this embodiment, a transistor Q₁₆ for short-circuiting the bias resistor R₇ is disposed across both ends of this resistor in order to prevent occurrence of the popping noise when the power source is turned on.

Emitters of differential transistors Q₃₅, Q₃₆ are connected to a stabilized power source voltage V_(cc3) via a resistor R₃₄, and resistors R₃₂, R₃₃ are connected to the collectors of these transistors, respectively. These transistors Q₃₅, Q₃₆ together form a switching circuit for alternatively actuating either one of the constant current transistors Q₅ and Q₁₂.

Namely, a voltage (V_(cc3) -2V_(BE)) generated by level shift diodes Q₃₇, Q₃₈ (the term "diode" will hereinafter include transistors of the diode type) and a resistor R₃₅ is applied to the base of the transistor Q₃₆. A diode Q₃₄ is connected to the emitter of the transistor Q₃₃ as a constant voltage element and a resistor R₃₀ is connected to its collector. A control voltage is applied to the base of this transistor Q₃₃, whose collector voltage is in turn applied to the base of the abovementioned transistor Q₃₅ via a resistor R₃₁. Accordingly, when the transistor Q₃₃ is kept on, the transistor Q₃₅ is turned on while the transistor Q₃₆ is turned off so that the constant current transistor Q₅ is rendered operative to transmit the input signal IN in the preamplifier 7. On the other hand, when the transistor Q₃₃ is kept off, the transistor Q₃₅ is turned off while the transistor Q₃₆ is turned on so that the constant current transistor Q₁₂ is rendered operative to carry out the muting operation in the pre-amplifier 7.

A resistor R₄₈ is interposed between a power source voltage V_(cc2), which is higher than the bias voltage V_(B1), and the collector of a transistor Q₃₂, and a control signal similar to the abovementioned control signal is applied to the base of the transistor Q₃₂, the collector voltage of which is in turn applied to the base of a switching transistor Q₁₆.

An intergration voltage of the power source voltage V_(cc) due to an integration circuit R₁₀₃, C₁₀₇ formed by external components at the eighth terminal is applied to the base of a transistor Q₂₃ via a resistor R₂₁, and level shift diodes Q₂₄, Q₂₅ and a resistor R₂₂ are connected in series to the emitter of this transistor Q₂₃. The voltage drop across this resistor R₂₂ generates the above-mentioned control signal that is to be impressed upon the base of the transistor Q₃₂. The terminal voltage of this resistor R₂₂ is applied to the base of transistor Q₃₂ via a resistor R₂₉ and to the base of transistor Q₃₃ via a resistor R₂₃, respectively.

The integrated voltage from this eighth terminal when the power source is turned on is utilized for eliminating the popping noise when the power source is turned on.

On the other hand, a detuning detection signal V_(M) generated by the aforementioned detuning detection circuit 4 is converted into a d.c. by an RC time constant circuit consisting of R₁₀₄, R₁₀₅ and a capacitor C₁₀₈, and is then applied to the seventh terminal. This detuning detection signal is applied to the base of a transistor Q₂₆ which is equipped with a diode Q₂₇ as a constant voltage element at its emitter and its collector is connected to the base of the abovementioned transistor Q₃₃.

Accordingly, the circuit including this transistor Q₂₆ forms the first mute control circuit 11.

The abovementioned detuning detection signal is applied to the second mute control circuit 12 via a resistor R₂₄ and the base and emitter of transistors Q₂₈, Q₂₉.

The second mute control circuit 12 employs: a Schmidt circuit consisting of a reference voltage circuit formed by differential transistors Q₃₉, Q₄₀ equipped with a resistor R₃₈ at their common emitters, a resistor R₄₁, a transistor Q₄₅ of the diode type, a diode Q₄₆, and voltage dividing resistors R₃₉, R₄₀ interposed between the collector and emitter of the transistor Q₄₅ for generating a reference voltage to be applied to the base of one (Q₄₀) of the transistors Q₃₉, Q₄₀. A current feedback circuit as a current mirror circuit is interposed between the collector and the base of the other transistor Q₃₉ and is composed of transistors Q₄₃, Q₄₄ and emitter resistors R₃₆, R₃₇. This Schmidt circuit has hysteresis with respect to an applied voltage which turns it on and an applied voltage which turns it off. Diodes Q₄₁, Q₄₂ for performing the voltage clamp are connected to the base of the abovementioned transistor Q₃₉ and the detuning detection signal obtained from the emitter of the transistor Q₂₉ is applied, via a resistor R₂₇, to an inverter circuit consisting of a transistor Q₃₀ and a resistor R₅₀. A transistor Q₃₁, which is controlled by the collector output of the transistor Q₃₀, is connected to the base of the transistor Q₃₉.

On the other hand, the output signal from the pre-amplifier 7 generated at the second terminal via the dielectric breakdown prevention resistor R₁₃ is applied to a PLL circuit 8 from the third terminal via a capacitor C₁₀₁. In the PLL circuit a 38 KHz switching signal for stereo demodulation in synchronism with 19 KHz pilot signal is formed, and is applied to the stereo demodulation circuit 9.

The stereo demodulation circuit 9 is composed of a known switching type stereo demodulation circuit. This stereo modulation circuit 9 consists of transistors Q₁₇ -Q₂₂ forming a balanced-differential type multiplication circuit and resistors R₁₆ -R₂₀. The stereo demodulation circuit 9 of this switching type was reported in the magazine "IEE TRANSACTIONS ON BROADCAST AND TELEVISION RECEIVERS", VOLUME BTR-14, NUMBER 3, p. 58-73, October, 1968, and a semiconductor integrated circuit for an FM stereo demodulation circuit using the PLL circuit was described in the aforementioned magazine "Electronics".

Reference numeral 13b represents a bias circuit for forming its bias voltage V_(B3).

The output of this stereo demodulation circuit 9 passes through post-amplifiers 14L, 14R so that the left channel output is obtained from the fourth terminal and the right channel output is obtained from the fifth terminal. Lowpass filters 15L, 15R are also connected to the fourth and fifth terminals, respectively. Capacitors C₁₀₃, C₁₀₄ and C₁₀₅, C₁₀₆ are connected in series and are also connected to the fourth and fifth terminals, respectively. Transistors Q₁₀₀, Q₁₀₁ are interposed between the respective junctions of these series capacitors and a ground potential terminal. These capacitors and transistors together form the muting circuit 10.

The bases of these transistors Q₁₀₀, Q₁₀₁ are connected to the sixth terminal via resistors R₁₀₁, R₁₀₂, respectively.

The collector voltage of transistor Q₃₆, which permits the muting operation in the pre-amplifier 7, is applied to the base of transistor Q₄₈ via a resistor R₄₄ and transistor Q₅₁ generates a mute control output signal. Diodes Q₄₉, Q₅₀ connected to the collector of the transistor Q₄₈ apply an operating voltage to this transistor Q₅₁. A resistor R₄₅ is interposed between the series diodes Q₄₉, 50 and the collector of the transistor Q₄₈, and a resistor R₄₆ is connected in parallel to the series diodes Q₄₉, Q₅₀.

On the other hand, the feedback output current of the abovementioned Schmidt circuit is also applied to the base of the output transistor Q₄₇, and a mute control output signal to the sixth terminal is generated via a resistor R₄₉ from the commonly connected collectors of the output transistors Q₅₁, Q₄₇.

The circuit of this embodiment operates in the following manner.

First, when the power source is turned on, transistor Q₃₂ is kept off and transistor Q₁₆ is kept on until the integration voltage to the eighth terminal reaches 4 V_(BE) (base-to-emitter voltage of the transistor or forward voltage of the diode). Accordingly, in response to the rising bias voltage which is following the power source voltage V_(cc1), the coupling capacitor C₁₀₀ charges and the bias voltage of the transistor Q₁ rises rapidly so that the bias voltage of the transistors Q₃, Q₁₁ generated by the feedback also rises rapidly.

On the other hand, during the period until the above-mentioned integration voltage reaches 5 V_(BE), the transistors Q₃₃ and Q₃₅ are kept off so that the constant current transistor Q₅ is turned off while the constant current transistor Q₁₂ is turned on. As the transistor Q₃₆ is turned on, the transistors Q₄₈, Q₅₁ are also turned on. Accordingly, the transistors Q₁₀₀ Q₁₀₁ of the muting circuit 10 are turned on and carry out the muting operation in the pre-amplifier 7 and in the muting circuit 10.

Before the integration voltage reaches 5 V_(BE), the transistor Q₃₂ is turned on and hence, the transistor Q₁₆ is turned off. When the integration voltage reaches 5 V_(BE), the transistors Q₃₃, Q₅ are turned on while the transistors Q₄₈, Q₅₁ are turned off so that the muting operation is released in the pre-amplifier circuit 7 and in the muting circuit 10.

Since in the rise of the bias voltage of the transistor Q₁ no delay is caused by the time constant determined by the capacitor C₁₀₀ and the resistor R₇, no difference is generated between the bias voltage V_(B1) and the bias voltage formed by the feedback operation. Consequently, the popping noise can be prevented.

On the other hand, when tuning to a desired station, the integration voltage at the eighth terminal is fully charged to the power source voltage V_(cc), so at the time of detuning the voltage at the seventh terminal is at the high level and transistor Q₂₆ is turned on. Accordingly, transistor Q₃₃ is turned off, whereby the muting operation is carried out in the pre-amplifier 7 and in the muting circuit 10. In this case, since the eighth terminal is at the high level, transistor Q₃₂ is turned on and the switching transistor Q16 connected to the bias resistor R₇ is turned off.

Since the seventh terminal is at the high level due to the abovementioned detuning detection signal, the transistor Q₃₉ forming the Schmidt circuit is turned on and the muting circuit 10 is also made to perform the muting operation by this mute control circuit 12.

When the detuning detection signal V_(M) drops below 2 V_(BE) upon tuning, the transistor Q₂₆ is turned off and the transistor Q₃₃ is turned on so that the muting operation in the pre-amplifier 7 is released. In this case, due to the S characteristic of the FM detection output, the voltage fluctuation occurs but is removed by the continued muting by muting circuit 10.

When the detuning detection signal V_(M) drops further below V_(BE), transistor Q₃₀ is turned off to thereby turn on transistor Q₃₁ so that the differential transistor Q₃₉ is compulsively turned off, and the action muting circuit 10 is released at this time.

When the detuning is again effected and the rise voltage of its detection signal V_(M) reaches V_(BE), transistor Q₃₀ is turned on, turning off transistor Q₃₁. In this case, however, a voltage higher than V_(BE), e.g., about 1.5 V_(BE), is applied to the base of the transistor Q₄₀ by the transistors Q₄₅, Q₄₆ and the resistors R₃₉, R₄₀, and consequently the transistor Q₃₉ is kept off and the muting circuit 10 does not operate yet.

When the detuning detection signal V_(M) reaches the abovementioned reference voltage (1.5 V_(BE)), transistor Q₃₉ is turned on, and muting is effected in the muting circuit 10.

In this manner, the mute control circuit 12 has the hysteresis characteristic having its discrimination levels at V_(BE) and 1.5 V_(BE).

Next, when the detuning detection signal reaches 2 V_(BE), transistor Q₂₆ is turned on to actuate transistor Q₃₃ and begin muting in the pre-amplifier 7. In this case, too, due to the S characteristic in the FM detection output, the voltage fluction occurs but is eliminated because the muting circuit 10 has already started operating.

In this embodiment, the hysteresis characteristic is provided by use of the Schmidt circuit because, as can be clearly understood from the operation waveform diagram of FIG. 9, no problem occurs even if the difference of timing between the start of the muting operation in the pre-amplifier 7 and the start of the muting operation in the muting circuit 10 at the beginning of the muting operation is made smaller than the difference of timing at the release of the muting operation. In other words, since the differential waveform in the d.c. voltage fluctuation of the S characteristic is produced as the popping noise, it is necesary for the timing difference to correspond to the pulse width of this popping noise in order to release the muting operation silently, and the timing of muting circuit 10 should be slightly advanced at the start of the muting operation. Additionally, the abovementioned hysteresis characteristic is an extremely effective means for compensating for unstability of the detuning detection signal.

The second invention of this application is not limited to the abovementioned embodiment in particular, and various circuit constructions may be employed for each circuit so long as the same operation can be provided.

Further, the monolithic semiconductor integrated circuit IC₂, forming the FM stereo demodulation circuit may be provided with additional circuits such as a circuit for FM stereo-monaural detection, selection, display and the like, a circuit for cancelling the 19 KHz pilot signal, and so forth.

Any signal can be used as the detuning detection signal so long as it is capable of discriminating between tuning and detuning, and the polarity of its signal level may become simultaneously high levels. In this case, the discrimination level may be set to the reverse level difference. 

What is claimed is:
 1. A muting circuit comprising:(1) first and second transistors having the emitters thereof connected to each other; (2) third and fourth transistors having the emitters thereof connected to each other; (3) a first constant current source connected commonly to the emitters of said first and second transistors; (4) a second constant current source connected commonly to the emitters of said third and fourth transistors; (5) a coupling capacitor for feeding an input signal to the base of said first transistor; (6) a bias resistor having one end thereof connected to the base of said first transistor and the other to the base of said third transistor; (7) bias voltage generation means connected to the base of said third transitor and to said other end of said bias resistor; (8) load means connected to the collector of said second transistor and to the collector of said fourth transistor; (9) a feedback circuit connected between said load means and both bases of said second and fourth transistors; (10) means for alternatively controlling and bringing either one of said first and second constant current sources into the operative state and the other into the inoperative state; and (11) a switching element connected between said one end and said other end of said bias resistor and actuated into the ON state for a predetermined period after making of the power source.
 2. The muting circuit as defined in claim 1 wherein said first constant current source includes a fifth transistor having the collector and emitter thereof connected to the emitters of said first and second transistors and to a reference potential, respectively; said second constant current source includes a sixth transistor having the collector and emitter thereof connected to the emitters of said third and fourth transistors and to said reference potential, respectively; and said switching element includes a seventh transistor having the collector-to-emitter path thereof connected in parallel to both ends of said bias resistor.
 3. The muting circuit as defined in claim 1 wherein a time constant circuit is connected between said power source and said referential potential and the bases of said fifth, sixth and seventh transistors are controlled on the basis of the output signal of said time constant circuit.
 4. An FM radio receiver comprising:(1) an FM intermediate frequency amplification stage; (2) an FM detector having the input terminal thereof connected to the output terminal of said FM intermediate frequency amplification stage; (3) a detuning detection circuit responsive to the intermediate frequency signal of said FM intermediate frequency amplification stage; (4) a pre-amplifier having the input terminal thereof connected to the output terminal of said FM detector, the signal transmitting operation of said pre-amplifier being virtually inhibited when a control input signal is applied to the control input terminal of said pre-amplifier; (5) a phase locked loop circuit for obtaining a 38 KHz switching signal in synchronism with a 19 KHz pilot signal contained in a stereo composite signal obtained from said FM detector; (6) a stereo demodulation circuit for providing right and left channel demodulation output signals when applied with said stereo composite signal and said 38 KHz switching signal; (7) a muting circuit having first and second input terminals, first and second output terminals and a control input terminal, said first and second input terminals receiving said right and left channel demodulation output signals, respectively, and said control input terminal receiving the control input signal, thereby virtually inhibiting the signal transmitting operation between said first input terminal and said first output terminal and the signal transmitting operation between said second input terminal and said second output terminal, respectively; (8) a first mute control circuit receiving at the input terminal thereof the output signal of said detuning detection circuit and applying the output signal thereof to said control input terminal of said pre-amplifier, said first mute control circuit further having a first discrimination level with respect to the output signal of said detuning detection signal to be applied to the input terminal thereof; and (9) a second mute control circuit receiving at the input terminal thereof the output signal of said detuning detection circuit and applying the output signal thereof to the control input terminal of said muting circuit, said second mute control circuit further having a second discrimination level with respect to the output signal of said detuning detection circuit to be applied to the input terminal thereof;whereby a level difference is set between said first discrimination level and said second discrimination level so that when the reception condition of said FM radio receiver changes from the tuning state to the detuning state, the signal transmitting operation of said muting circuit is virtually inhibited and thereafter the signal transmitting operation of said pre-amplifier is virtually inhibited.
 5. The FM radio receiver as defined in claim 4 wherein there is further disposed a detection circuit for detecting the turning on of the power source, and the signal transmitting operation of said pre-amplifier and that of said muting circuit are virtually inhibited on the basis of the output signal of said detection circuit for detecting the turning on of the power source.
 6. The FM radio receiver as defined in claim 4 or 5 wherein said second discrimination level of said second mute control circuit is determined by the hysteresis of a Schmidt circuit, and a level difference is provided between the start level of virtual inhibition of the signal transmitting operation of said Schmidt circuit and the release level of virtual inhibition of the signal transmitting operation of said muting circuit.
 7. The FM radio receiver as defined in claim 4 or 5 wherein all of said pre-amplifier, said phase locked loop circuit, said stereo demodulation circuit, said first mute control circuit and said second mute control circuit are formed inside one monolithic semiconductor integrated circuit. 